1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotography recording scheme, such as a laser printer, a copier and a facsimile machine.
2. Description of the Related Art
Some electrophotography image forming apparatuses employ a method of repeating multiple times a process of transferring, onto a recording sheet, a toner image having been formed on an image carrier by charging, exposing and developing, thereby forming an image in which multiple colors are overlapped with each other on the recording sheet to acquire a color image. In such a color image forming apparatus, a phenomenon occurs where a gap of white that should not exist is formed between adjacent different colors of images. This is because, when a latent image with abrupt change in drum surface potential, e.g. an image edge portion, is formed on the photosensitive drum, a developing device develops the area such that a visualized image is formed narrowly in comparison with the electrostatic latent image originally formed on a photosensitive drum. In the case of monochrome image formation, there is no adjacent color. Accordingly, a possible narrowing to a certain extent, which may occur in an image, causes no problem. However, image formation in such a state causes the following phenomenon. For instance, in the case of an image where a cyan band and a black band are adjacent to each other, each of a cyan visualized image and a black visualized image is narrowly formed; the image is to be formed such that the cyan band and the black band are adjacent to each other with no gap. Accordingly, a finally transferred image on a sheet unfortunately has a gap between a cyan part and a black part. Such a phenomenon will hereinafter be called a white gap.
FIG. 10A is a diagram illustrating a white gap according to a conventional technique in detail. A phenomenon where a visualized image (visualized area) is narrowed occurs because the electric field is wrapped at edge areas of an electrostatic latent image (latent image area) formed on a photosensitive drum 1a as illustrated in the diagram. Japanese Patent Application Laid-Open No. 2003-312050 discloses a technique that is called non-image-area exposure and, in an image forming unit, a light emitting element of a laser scanner is caused to emit weak light with a small light amount onto the entire surface of a printable region to an extent preventing redundant toner from adhering, thereby preventing an image from being narrowed. More specifically, a method is employed that is called a pulse width modulation (PWM) scheme and changes the duty ratio of a pulse wave. This scheme causes the light emitting element of the laser scanner to emit light with a pulse having a very short pulse width corresponding to an emission amount of weak light.
The non-image-area exposure as described above is employed not only for measures against a white gap but also for stabilizing (properly adjusting) the voltage of the surface of a charged photosensitive drum 1a to suppress an image failure in a printable region.
However, in the configuration of the conventional image forming apparatus, a non-image-area exposure region irradiated with weak light intensity is the same as a printable region. Accordingly, a phenomenon sometimes occurs where toner adheres to a place in a sheet edge area not to be developed. Such a phenomenon is called fogging. Referring to FIGS. 10B and 10C, specific description will be made. Conventionally, a signal for weak light intensity emission for non-image-area exposure is processed as with an image signal. That is, a signal for driving a laser diode is subjected to pulse width modulation to adjust laser emission time. Accordingly, as illustrated in FIG. 10B, in the longitudinal direction (laser main scanning direction) that is the rotational axis direction of a photosensitive drum, which is an image carrier, in terms of a light emission region, a printable region (image-area exposure region) where a laser is on for printing is the same as a non-image-area exposure region where the laser is emits as weak emit light with a small light amount. In both the longitudinal direction (main scanning direction) (longitudinal position) and the sheet conveying direction (sub-scanning direction), a part with a prescribed distance apart from a sheet edge is regarded as a print-prohibition region. The printable region is thus set smaller than an actual sheet size to prevent an image from protruding from the actual sheet size. That is, a margin is set as a print-prohibition region.
Accordingly, as illustrated in FIG. 10B, in the case where the printable region is the same as the non-image-area exposure region, the following occurs. That is, the absolute value of the charged voltage (called a photosensitive drum voltage) (Vd) (illustrated with no non-image-area exposure) on the surface of the photosensitive drum in a print-prohibition region provided outside the printable region is larger than the absolute value of the photosensitive drum voltage of the non-image-area exposure region (Vdbg) (illustrated with non-image-area exposure). Accordingly, in the print-prohibition region, a background contrast Vback (=|Vd|−|Vdc|), which is the contrast between a developing voltage Vdc and a photosensitive drum voltage Vd, is unfortunately larger than the printable region (here, Vback=|Vdbg|−|Vdc|). Thus, in the print-prohibition region, fogging (reversed fogging) occurs owing to toner (reversed toner) charged in a polarity reversed to the original polarity.
The reason will be described below. As illustrated in FIG. 10C, the photosensitive drum voltage (Vd) is set relatively high in consideration of the variation in the exposure area voltage (VL). Repetitive image forming operations (sheet feeding) successively expose the photosensitive drum to cause residual charge. Accordingly, in a state after feeding sheet, the VL voltage is increased (VL UP) in comparison with the initial state. The relative high setting is made for addressing the increase. Thus, the developing voltage (Vdc) is controlled such that the developing contrast Vcont (=|Vdc|−|VL|) is maintained constant to compensate increase in exposure area voltage (VL) of the photosensitive drum, thereby preventing the density of the image from varying. Meanwhile, even under such control of the developing voltage maintaining the developing contrast (Vcont), if the initial photosensitive drum voltage (Vd) is constant, the background contrast (Vback) after feeding sheet is unfortunately reduced. To address the reduction, the initial photosensitive drum voltage (Vd) is preset relatively higher, the non-image-area exposure is performed on the non-image-area exposure region to reduce the voltage of the photosensitive drum from a value Vd to an appropriate value Vdbg. Accordingly, the optimal background contrast (Vback) is maintained, thereby preventing fogging from occurring. However, the photosensitive drum voltage (Vd) on a region without non-image-area exposure becomes excessive, and reversed fogging tends to easily occur. Particularly, in this phenomenon, the laser is caused to always emit light in the case of non-image exposure. Accordingly, the VL voltage tends to significantly increase. In a high temperature and high humidity environment, the photosensitive drum voltage (Vd) is high and the toner charge is low. Accordingly, fogging tends to easily occur.
To reduce fogging, weak light with a small light amount exposure on the entire charged region may be considered. However, the weak light exposure with a small light amount on the entire charged region unfortunately increases laser emission time, and causes a possibility of reducing the life of the laser.